1. Field of the Invention
The present invention relates to a hard film, a sliding part, and a method of manufacturing the sliding part.
2. Description of the Related Art
Techniques for reducing the friction of sliding parts have been intensively studied to contribute to reduction of fuel consumption in internal combustion engines and to improvement in the working life of parts. Their typical examples are development for the solid lubrication films and development of texturing allowing a desired concavo/convex shape to be formed on the surface. Solid lubrication films exhibit satisfactory effects in a circumstance with no lubrication and, in contrast, the texturing, which uses a structure in which lubrication oils are accumulated, effectively functions in a circumstance where the lubrication oils are used.
Further, by combining the techniques described above, a technique in which the solid lubrication film and the texture coexist has been devised. Such a combined technique is applied to a sliding circumstance where the thickness of the oil membrane of the lubrication oil changes or to a lubrication circumstance where the thickness of the oil membrane is thin and a solid contact is innegligible.
A technique of providing a concavo/convex shape to the surface of a film has been developed as in JP-2002-235852-A and JP-H06 (1994)-41721-A. According to JP-2002-235852-A, a film having a plateau shape at the surface prevents oil run out and exhibits a low friction property in a lubrication oil. According to JP-H06 (1994)-41721-A, a hard film having a truncated concavo/convex shape ensures oil retention and thus has anti-galling property and wear resistance.
Meanwhile, DLC (Diamond-like Carbon) is a typical material for a solid lubrication film. DLC is well-known as a film adapted to allow high hardness and self-lubrication property to coexist. DLC is a film which is a material also referred to as amorphous carbon, a-C (amorphous carbon), a-C:H (hydrogenated amorphous carbon), i-C (i-carbon), and hard carbon. The film deposition method of DLC includes a PVD method or a plasma CVD method such as sputtering and arc ion plating, wherein when a hydrocarbon gas is used for the starting material, a carbon film containing hydrogen is formed.
The micro structure of DLC is an amorphous structure including both sp2 bonding and sp3 bonding as bonding of carbon atoms to each other but not having a distinct crystal structure (not having grain boundary). DLC can provide high hardness and high toughness in comparison with a crystalline hard film made of TiN (titanium chloride), TiAlN (titanium aluminum nitride), and CrN (chromium nitride), or the like. DLC has a low friction coefficient since it has self-lubricity derived from the sp2 bonding.
However, DLC suffers from deterioration of film quality such as a decrease in film strength or formation of defects on the surface in a temperature region of 300° C. or higher, with DLC having heat resistant temperature that is remarkably low compared with about 500° C. of TiN and about 800° C. of TiAlN. Since the friction surface is at a high temperature state locally due to heat of friction, DLC may sometimes be worn remarkably depending on the sliding condition.
A film with addition of other elements to DLC has been developed with an aim of improving the wear resistance of DLC and JP-2011-26591-A describes DLC containing 5 to 25 atomic % of hydrogen and 4 to 30 atomic % of boron at the surface of a member used in a lubrication oil.
With an aim of ensuring low friction property in a no-lubrication state, a molybdenum additive is blended to a lubrication oil in JP-2002-235852-A, while sulfur, phosphorus, zinc, calcium, magnesium, sodium, barium, and copper are blended each in an appropriate amount to the lubrication oil in JP-H06 (1994)-41721-A.
Further, with an aim of improving the heat resistance, a film with addition of boron and nitrogen together to DLC has also been developed. According to JP-2004-60668-A, a rolling device are improved in anti-galling property and wear resistance by forming a B—C—N layer comprising amorphous carbon containing boron and nitrogen.
According to JP-2005-282668-A, a fastening jig has a sliding surface'coated with a film comprising from 40 atomic % or more to less than 99 atomic % of carbon and from 1 atomic % or more and less than 40 atomic % of hydrogen where carbon is substituted with boron and nitrogen in a range of 10 to 80%. Such a fastening jig exhibits satisfactory anti-galling property.